This invention relates to an air electrode having discrete, small, high surface area oxide particles on its exterior surface which serve as nucleating sites for subsequently applied solid electrolyte.
High temperature solid oxide fuel cell configurations are well known, and taught, for example, in U.S. Pat. No. 4,490,444 (Isenberg), herein incorporated by reference. There, a porous, calcia stabilized zirconia support tube, having a porous air electrode of, for example calcium, strontium, magnesium or zirconium oxide doped lanthanum manganite was taught, with an attached, axially elongated, narrow interconnection strip of calcium, strontium, or magnesium oxide doped lanthanum chromite. The air electrode was coated with a 20 micrometer to 50 micrometer thick, solid, non-porous, yttria stabilized zirconia electrolyte. A porous, nickel-zirconia cermet, exterior fuel electrode, about 50 micrometers thick, covered most of the electrolyte. In another embodiment, taught in U.S. Pat. No. 4,547,437 (Isenberg et al.), an electrode-protective, porous, continuous interlayer of calcium and cobalt doped yttrium chromite was disposed between the air electrode and the electrolyte. Also, Bergmann et al., in Extended Abstracts Of Presentations At Workshop On High Temperature Solid Oxide Fuel Cells, "Transport Considerations In Oxygen Electrodes Of The Triphase Boundary Type For Zirconia Cells" May 1977, Brookhaven National Laboratory, taught complete, continuous separation layers of ceria or doped zirconia between silver, platinum or indium oxide electrodes and zirconia electrolyte, in order to decrease polarization losses.
In U.S. Pat. No. 4,562,124 (Ruka), cerium was incorporated into the atomic structure of the air electrode to provide the composition of La.sub..3 Ca.sub..5 to .6 Ce.sub..1 to .2 MnO.sub.3. The addition of cerium helped match the coefficient of thermal expansion of the air electrode to the support tube and the electrolyte. For a variety of reasons, cerium compounds have also been applied to fuel electrodes of electrochemical cells, as an impregnated material, as in U.S. Pat. No. No. 4,894,297 (Singh et al.) and as an exterior particulate film, as in U.S. Pat. No. 4,885,078 (Spengler et al.)
In conventionally fabricated tubular fuel cells, electrolyte penetration within the air electrode and encapsulation of the air electrode surface by the electrolyte film has been observed near the air electrode-electrolyte interface. After electrical testing, the air electrode of these cells have been found to show structural changes in terms of porosity-formation and densification. Such undesirable structural changes taking place in the air electrode near the air electrode-electrolyte interface are postulated to be due to changes in the oxygen stoichiometry of the air electrode, resulting from the partial encapsulation of the air electrode particles at the air electrode-electrolyte interface. Partially encapsulated air electrode surfaces formed near the electrolyte-air electrode interface may also inhibit oxygen reduction reaction due to limiting the surface area for electron exchange at the interface, and allow oxygen loss from the air electrode lattice during cell operation at moderate-to-high current densities.
One of the main objects of this invention is to reduce oxygen loss from air electrode particles in contact with the electrolyte and increase the active area for the electron exchange reactions with oxygen at the electrode-electrolyte interface.